1. Field of the Invention
The present invention relates to work-clamp pallets for various machine tools of the vertical or the horizontal type, especially those for NC machine tools of the vertical or the horizontal type.
2. Description of the Prior Art
For machining on a machine tool, work pieces are required to be precisely positioned on a work table and subsequently clamped securely thereto.
Especially in the case of an NC machine tool, no truly precise machining of work pieces can be accomplished unless they are position precisely.
While in recent years there has been remarkable progress in performance of NC machine tools such as cutting speed, the variety and performance of cutting tools and the automatic changing of cutting tools, resulting in a marked improvement of their working efficiency. However, there is still much to be desired about the improving the means for changing work pieces since this has been a major bottle-neck in the desired improvement of their working efficiency and productivity.
Hitherto, in a vertical machine tool it has been a usual practice to provide T-sectioned grooves in a work-clamping face of a work table and to position one or more workpieces directly on the work table. After subsequent clamping of the positioned workpieces by means of bolts or small clamping devices, the position of each workpiece is checked by actual measurement. In many cases it is necessary to adjust and re-clamp each workpiece before proceeding with the machining thereof.
According to this conventional practice, however, the machine tool has to be stopped while the finished workpieces are removed and new workpieces for the next cycle of machining are set on the work table. Hence, even with a modern NC machine tool, the net working rate is then 30-40% at the best, the remaining 60-70% of the working hours are lost in preparatory work such as changing workpieces and cleaning the machine for removal of chips.
For improving the efficiency of the work-changing procedure, therefore, studies have been made of various clamping means and methods of placing them properly arranged on the work table.
For instance, an attempt was made to have fixedly secured to the topside of the work table a work-clamp base having arranged on it a plurality of hydraulic clamping devices. The hydraulic cylinder of each of these clamping devices was connected to a hydraulic pressure supply so that a plurality of workpieces could be clamped quickly.
Since, however, the abovementioned work-clamp base is fixedly secured to the work table of a machine tool, the changing of workpieces is made on the work table also. So, while the method is effective indeed for improving the efficiency of the work-changing procedure, very slight improvement is made in the machine's working efficiency.
For overcoming this problem, the idea was proposed of using a work-clamp pallet which allows changing of workpieces outside the machine tool.
According to the idea, a pallet-changing table is provided in front of a machine tool so as to enable exchange of work-clamp pallets between it and the machine tool's work table. Works are set on a plurality of work-clamp pallets, one at a time, at the work-changing station of the pallet-changing table, each work-clamp pallet so prepared is transferred onto the work table and positioned thereon and clamped thereto to be thus readied for machining of the workpieces set thereon.
When the abovementioned work-clamp pallet, however, problems arise is with the hydraulic clamping devices.
When each work-clamp pallet is transferred with the hydraulic hoses leading from the hydraulic pressure supply device connected thereto, those hoses are required to be long and the multiplicity thereof being entangled interferes with smooth transfer of the pallet.
Moreover, during machining, the work table with the work-clamp pallet clamped thereon moves longitudinally as well as laterally over substantial strokes, hence means have to be provided for reeling the hydraulic hoses in and out lest they interfere with movement of the work table.
Also, if any of the hydraulic hoses should be burnt by the red-hot chips resulting from machining, the resulting leakage of hydraulic oil results in malfunctioning of the clamping devices and, machining must be interrupted for replacing the work-clamp pallet. It is also possible that the leaked hydraulic oil may be ignited to cause a fire which might develop to be a catastrophe involving the entire plant.
In order to cope with the above problem, it is possible to shut off the oil path by a directional changeover valve after feeding hydraulic pressure to the hydraulic clamping devices for clamping the workpieces and thereafter transfer the work-clamp pallet to the NC machine with the hydraulic hoses disconnected.
With a spool valve ordinarily used as such a directional changeover valve, however, perfectly leak-free shutting-off of the hydraulic oil is impossible. Hence, hydraulic pressure leakage from the hydraulic clampine devices is inevitable.
Should a hydraulic pressure leakage occur, the resultant decrease of the clamping force is likely to cause false machining of the workpieces and even if this could be avoided, it is required to interrupt machining for replacing the work-clamp pallet.
This risk can be precluded if the hydraulic hoses are not disconnected after clamping of the work-clamp pallet to the work table, but this is accompanied by the abovementioned problem.
Thus, the idea of providing the work-clamp pallet with hydraulic clamping devices has not yet been put to practical use since it is accompanied by a number of problems or difficulties, and is also expensive.
Providing the work-clamp pallet with mechanical clamping devices using a screw mechanism or the like, on the other hand, is also accompanied by drawbacks.
With a mechanical clamping device miniaturizing is difficult, as is generation of the required clamping force of say several tons. Another problem is that mechanical clamping devices have to be manipulated one after another for clamping as well as unclamping, hence no substantial improvement of the efficiency of work-setting procedure can be obtained.
The work-clamp pallet is put to wider practical use for machining tools of the horizontal type than for the vertical type.
Today many of the latest, large horizontal machining centers are complete with a multi-pallet automatic pallet-changing device for automatic changing of workpieces.
The work-clamp pallet used in this case is box-shaped with work-clamping faces provided on 4 or 2 sides thereof. The clamping device employed therefor has an improved universal feature, for workpieces to be machined in this case are relatively large, and varied in shape.
Each work-clamping face thereof, is provided with a multiplicity of tapped holes and is so designed such that workpieces are clamped to the work-clamping face by a plurality of bolts.
With this type of work-clamp pallet positioning of workpieces on the work-clamping face is not easy, the work-clamping procedure being somewhat troublesome. Hence, it is less suited for clamping small works.